The invention concerns gas turbines and in particular gas turbines of aircraft turbojet engines. It concerns more specifically a sealing ring to be placed around a wheel of a gas turbine to limit gas leaks at the ends of the blades.
In general, the cooling of the turbine elements in contact with the propelling gases permits an increase in the combustion temperature and consequently of the power and yield of the turbojet engine. In regard to the gasket seals made of a material capable of being abraded by the ends of the blades, it has already been proposed to cool them by means of the forced circulation of a fraction of the flow of air delivered by the compressor through the pores or interstices of said material. U.S. Pat. No. 3,825,364 describes, for example, a sealing ring comprising in succesion from the periphery toward the axis:
(a) an annular support placed around the wheel,
(b) a first annular layer (to be designated for the moment as the "distributing layer"), attached to said annular support and made of a material permeable to air,
(c) a second annular layer (to be designated the "wear layer") secured to the inside of the distributing layer, extending to the immediate vicinity of the blades of the wheel and made of a porous material with open pores, capable of being abraded by ends of the blades and unaffected by the thermaland chemical action of the gases driving the wheel.
The annular support is designed so as to form an annular passage around the distributing layer. Orifices provided in the annular support act to divert a fraction of the compressor air flow toward the annular passage. The air permeability of the wear layer is greater than that of the distributing layer which is supposed to distribute the cooling air homogeneously through the wear layer. The fact that the latter is designed to be more permeable than the distributing layer is intended to maintain the air flow at its initial value even when wear by the ends of the blades leads to a partial obstruction of the superficial pores of the wear layer.
This process of cooling the wear layer by the circulation of air through the layer in actual fact has significant disadvantages whereby the device of the prior art described hereinabove attains its object in part only. In fact,
in order for the air to flow at an adequate rate in spite of the pressure prevailing in the jet it is necessary to force it around the distributing layer under a relatively high pressure (for example, of the order of 15 bars); this affects the performance of the turbojet engine;
because of the variation of gas pressure during its passage through the wheel, the distribution of the air flow through the wear layer is not as homogeneous as might be expected; the air moves in the layer not only in the radial direction (i.e., transversely to the layer), but also axially (i.e., parallel to the axis of the wheel) and the axial flow opposes the radial flow;
even though, when the gasket is new, the permeability of the wear layer is greater than that of the distributing layer, the superficial pores or interstices of the wear layer become obstructed more or less in the long run, by pollution and by "coating" and the cooling air flow may decrease dangerously;
the choice of the properties of the two layers are closely related to each other, because the cooling air passes through both, but its flow rate must remain limited by the cooling layer; the selections thus are the result of a compromise;
finally, the gasket is thicker than the gaskets of the prior art, because the thickness of the wear layer is added to that of the distributing layer and must be of a sufficient thickness to be able to play its role as a distributor of air.